1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system which is suitable as an imaging optical system for use mainly with an imaging sensor, the high zoom-ratio zoom lens system having a wide angle-of-view of a half angle-of-view of approximately 35 degrees.
2. Description of Related Art
In recent years, there has been an increased need for digital cameras that have a high zoom-ratio zoom lens system having a zoom ratio of 10:1 or more. Furthermore, there is a strong demand for a wider angle-of-view; high zoom-ratio zoom lens systems having an angle-of-view of 70 degrees or more have become popular. In addition, further advancements in the miniaturization of the pixel pitch have been made, requiring the optical system to have a higher resolution. In an imaging lens system that is for use mainly in a compact camera, it is common for such a compact camera to achieve a slim design via the employment of a retractable barrel mechanism, in which the air-distances between lens groups are reduced at a lens barrel retracted state (accommodated state). However, when designing a zoom lens system which is suitable for such a retractable barrel mechanism, in addition to reducing the thickness of each lens group in a lens system, it is also necessary for the frontmost lens to be small in diameter, and furthermore, for this lens system to be designed with consideration of the mechanical restrictions in regard to the overall length thereof.
Japanese Unexamined Patent Publication Nos. 2008-292733 and 2009-42269 each discloses a zoom lens system which achieves a zoom ratio of approximately 15:1 through 20:1. However, in the zoom lens system of the above-mentioned Japanese Unexamined Patent Publication No. 2008-292733, although the zoom ratio is sufficient, the angle-of-view is only approximately 60 degrees, and hence does not satisfy the specifications required for a wide angle-of-view. In regard to the zoom lens system that is disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2009-42269, although this zoom lens system achieves a high zoom ratio and a wide angle-of-view, the overall length of the zoom lens system at the long focal length extremity is either longer or approximately the same as the focal length thereof, so that such a zoom lens system does not sufficiently achieve miniaturization.
In addition, an image-stabilizing function is demanded in a high zoom-ratio zoom lens system, and hence, it becomes an important issue to maintain sufficient optical quality during image-stabilizing operations while also achieving a miniaturized zoom lens system. For example, in Japanese Unexamined Patent Publication No. 2003-140048, an image-stabilizing mechanism is disclosed in which a zoom lens system having a positive first lens group, a negative second lens group, a positive third lens group and a positive fourth lens group, the third lens group thereof is divided into a positive first sub-lens group and a negative second sub-lens group; the angle of incidence of the light rays onto the second sub-lens group is set to be substantially parallel to the optical axis, and deterioration in the optical quality due to image-stabilization is suppressed (corrected) by moving only the second sub-lens group in directions orthogonal to the optical axis. However, since this image-stabilizing mechanism divides the third lens group into two sub-lens groups, the mechanical arrangement therefor is complex, and the optical unit thereof is unavoidably and undesirably enlarged.